image: Add a new API.

Raw disk-images and ISO9660 images are created in a Qemu virtual machine. This
is quite fragile, very slow, and almost unusable without KVM.

For all these reasons, add support for host image generation. This implies the
use new image generation mechanisms.

- Raw disk images: images of partitions are created using tools such as mke2fs
  and mkdosfs depending on the partition file-system type. The partition
  images are then assembled into a final image using genimage.

- ISO9660 images: the ISO root directory is populated within the store. GNU
  xorriso is then called on that directory, in the exact same way as this is
  done in (gnu build vm) module.

Those mechanisms are built upon the new (gnu image) module.

* gnu/image.scm: New file.
* gnu/system/image.scm: New file.
* gnu/build/image: New file.
* gnu/local.mk: Add them.
* gnu/system/vm.scm (system-disk-image): Rename to system-disk-image-in-vm.
* gnu/ci.scm (qemu-jobs): Adapt to new API.
* gnu/tests/install.scm (run-install): Ditto.
* guix/scripts/system.scm (system-derivation-for-action): Ditto.

gnu/tests/install.scm

@@ -22,9 +22,11 @@
 (define-module (gnu tests install)
   #:use-module (gnu)
   #:use-module (gnu bootloader extlinux)
+  #:use-module (gnu image)
   #:use-module (gnu tests)
   #:use-module (gnu tests base)
   #:use-module (gnu system)
+  #:use-module (gnu system image)
   #:use-module (gnu system install)
   #:use-module (gnu system vm)
   #:use-module ((gnu build vm) #:select (qemu-command))
@@ -229,14 +231,18 @@ packages defined in installation-os."
                        ;; we cheat a little bit by adding TARGET to its GC
                        ;; roots.  This way, we know 'guix system init' will
                        ;; succeed.
-                       (image  (system-disk-image
-                                (operating-system-with-gc-roots
-                                 os (list target))
-                                #:disk-image-size install-size
-                                #:file-system-type
-                                installation-disk-image-file-system-type
-                                ;; Don't provide substitutes; too big.
-                                #:substitutable? #f)))
+                       (image
+                        (system-image
+                         (image
+                          (inherit
+                           (find-image
+                            installation-disk-image-file-system-type))
+                          (size install-size)
+                          (operating-system
+                            (operating-system-with-gc-roots
+                             os (list target)))
+                          ;; Don't provide substitutes; too big.
+                          (substitutable? #f)))))
     (define install
       (with-imported-modules '((guix build utils)
                                (gnu build marionette))